Device for temperature measurement

ABSTRACT

A device for temperature measurement uses an optical system to image the heat radiation emanating from a measurement spot on an object of measurement onto a detector. A sighting arrangement is also provided which has a diffractive optical system by which a light intensity distribution is produced which corresponds to the position and size of the measurement spot on the object of measurement.

BACKGROUND OF THE INVENTION

[0001] Device for temperature measurement

[0002] The invention relates to a device for temperature measurement.

[0003] Such devices which are known in the art for contactlesstemperature measurement comprise a detector for receiving heat radiationemanating from a measurement spot on an object of measurement, anoptical system for imaging the heat radiation emanating from themeasurement spot onto the detector and a sighting arrangement foridentifying the position and size of the measurement spot on the objectof measurement by means of visible light. A further processingarrangement which converts the detector signal into a temperatureindication is also connected to the detector.

[0004] In this case the optical system is so designed that at a certainmeasurement distance for the most part only heat radiation from acertain area of the object of measurement, namely the so-calledmeasurement spot, is focussed onto the detector. In most cases the sizeof the measurement spot is defined by the area from which 90% of theheat rays focussed onto the detector strike. However, applications arealso known in which there are reference to values between 50% and 100%.

[0005] The pattern of the dependence of the size of the measurement spotupon the measurement distance depends upon the design of the opticalsystem. A fundamental distinction is made between distant focussing andclose focussing. In distant focussing the optical system images thedetector into infinity and in close focussing it images it onto thefocus plane. In the case of distant focussing it is necessary to dealwith a measurement spot which grows linearly with the measurementdistance, whereas in close focussing the measurement spot will first ofall become smaller with the measurement distance and after the focusplane will enlarge again if the free aperture of the optical system isgreater than the measurement spot in the focus plane. If the measurementspot in the focus plane is greater than the free aperture of the opticalsystem, then the measurement spot is also enlarged with the measurementdistance even before the focus plane. Only the increase in the size ofthe measurement spot is smaller before the focus plane than after it.

[0006] In the past various attempts were made to render the position andsize of the measurement spot, which is invisible per se, visible byillumination. According to JP-A-47-22521 a plurality of rays whichoriginate from several light sources or are obtained by reflection froma light source are directed along the marginal rays of a close-focussedoptical system onto the object of measurement. In this way the size andposition of the measurement spot for a close-focussed system can berendered visible by an annular arrangement of illuminated points aroundthe measurement spot. U.S. Pat. No. 5,368,392 describes various methodsof outlining measurement spots by laser beams. These include themechanical deflection of one or several laser beams as well as thesplitting of a laser beam by a beam divider or a fiber optic system intoseveral single beams which surround the measurement spot.

[0007] A sighting system is also known in the art which uses two laserbeams to describe the size of the measurement spot. This system uses twodivergent beams emanating from the edge of the optical system tocharacterise a close-focussed system and two laser beams which intersectin the focus point to characterise a close-focussed optical system.

[0008] All known sighting arrangements are either only useful for acertain measurement distance or require relatively complex adjustmentand are often quite expensive.

SUMMARY OF THE INVENTION

[0009] The object of the invention, therefore, is to make furtherdevelopments to the device for temperature measurement in such a way asto facilitate simple identification of the position and size of themeasurement spot independently of the distance.

[0010] This object is achieved according to one aspect of the invention,in that the sighting arrangement has a diffractive optical system forproducing a light intensity distribution with which the position andsize of the measurement spot on the object of measurement can berendered visible.

[0011] According to another aspect of the invention, a diffractiveoptical system is an optical element, the function of which is basedprincipally upon the diffraction or light waves. In order to produce thediffraction, transverse microstructures which can consist, for example,of a surface profile or a refractive index profile are provided in theoptical element. Diffractive optical elements with a surface profile arealso known as so-called holographic elements. The surface patterns areproduced for example by exposure of photoresist layers to light andsubsequent etching. Such a surface profile can also be converted byelectroplating into an embossing printing block with which the hologramprofile can be transferred into heated plastic films and reproduced.Thus many holographic elements can be produced economically from onehologram printing block.

[0012] The pattern of the diffractive optical system is produced byinterference of an object wave with a reference wave. If for example aspherical wave is used as the object wave and a plane wave as thereference wave then an intensity distribution is produced in the imageplane which is composed of a point in the centre (0^(th) order), a firstintensive circle (first order) and further less intensive circle ofgreater diameter (higher orders). By screening out of the 0^(th) and thehigher orders an individual circle can be filtered out. A plurality ofother intensity distributions which are explained in greater detailbelow with reference to several embodiments can be produced by otherobject waves.

[0013] According to another aspect of the invention, usuallyapproximately 80% of the energy emanating from the light source lies inthe patterns produced by the diffractive optical system. The remainingenergy is distributed inside and outside the measurement spot.

[0014] According to a further aspect of the invention, the lightintensity distribution which is produced can be formed, for example, bya circular marking surrounding the measurement spot or a cross-shapedmarking.

[0015] Such a device can also be produced economically and only requiresa little adjustment work.

[0016] Further constructions of the invention are the subject of thesubordinate claims and are explained in greater detail below withreference to the description of several embodiments and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 shows a schematic representation of a device according tothe invention for temperature measurement according to a firstembodiment;

[0018]FIGS. 2a to 2 g show schematic representations of various lightintensity distributions for identifying the position and size of themeasurement spot;

[0019]FIG. 3 shows a schematic representation or a device according tothe invention for temperature measurement according to a secondembodiment;

[0020]FIG. 4 shows a schematic representation of a device according tothe invention for temperature measurement according to a thirdembodiment;

[0021]FIG. 5 shows a schematic representation of a device according tothe invention for temperature measurement according to a fourthembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022]FIG. 1 shows a first embodiment of a device according to theinvention for temperature measurement, comprising

[0023] (a) a detector 1 for receiving heat radiation 3 emanating from ameasurement spot 2 a of an object of measurement 2,

[0024] (b) an optical system 4 for imaging the heat radiation emanatingfrom the measurement spot 2 a onto the detector 1,

[0025] c) and a sighting arrangement 5 for identifying the position andsize of the measurement spot 2 a on the object of measurement 2 by meansof visible light 6.

[0026] The sighting arrangement 5 consists essentially of a light source5 a, a diffractive optical system formed for example by a holographicelement 5 b and an additional refracting and/or reflecting opticalelement 5 c. The light source 5 a sends a reference wave 6 a onto theholographic element 5 b, resulting in a conically opening hologram 6 bwhich is transformed by the optical element 5 c so that it forms anintensity distribution 6 c which describes the position and size of themeasurement spot 2 a over all measurement distances.

[0027] A laser is advantageously used as the light source 5 a forgenerating the reference wave. However, it is also possible to use asemiconductor light-emitting diode or a thermal light source. When athermal light source is used a filter is advantageously provided inorder to reduce the chromatic aberrations.

[0028] The optical system 4 is formed by a dichroic beam divider 4 a andan infrared lens 4 b. The heat radiation 3 emanating from themeasurement spot 2 a first of all reaches the beam divider 4 a whichdeflects the heat radiation, i.e., the infrared radiation, by 90° anddelivers it to the infrared lens 4 b .

[0029] Since the beam divider 4 a must of necessity lie in the beam pathof the sighting arrangement 5 it is constructed as a dichromatic beamdivider which is reflective for the heat radiation emanating from themeasurement spot 2 a and transparent for the visible light of thesighting arrangement 5.

[0030] The size of the marking to be produced depends essentially upontwo parameters, namely the measurement distance and the desired accuracyof measurement. The accuracy of measurement results from the percentageof the rays emanating from the measurement spot and focussed onto thedetector. The area of the measurement spot can for example be defined bythe fact that 90% of the emanating radiation reaches the detector.However, depending upon the application this percentage can also bechanged.

[0031] The optical element 5 c which is adapted to the optical system 4is provided in order to ensure that in each measurement distance themarking produced for identifying the measurement spot has the correctsize for the desired accuracy or measurement.

[0032]FIGS. 2a to 2 g show light intensity distributions such as mightbe produced on the object of measurement 2 for identifying themeasurement spot 2 a. FIGS. 2a to 2 d show annular markings whichsubstantially outline the measurement spot 2 a. In this case themarkings can be configured as in FIGS. 2a and 2 c as a closed circle 3 aof in FIGS. 2b and 2 d as a broken circle 3 b. It may also beadvantageous to represent the centre of the measurement spot by afurther marking 3 c, for example in the form of a dot.

[0033] In FIGS. 2e and 2 f the light intensity distributions arerepresented as cross-shaped markings 3 d and 3 e respectively. In thiscase the point of intersection represents the centre of the measurementspot 2 a and the four corner points represent the outer limits thereof.

[0034] A very advantageous light intensity distribution is representedin FIG. 2g in the form of a plurality of concentric circles 3 f, 3 g, 3h. In this case each circle represents a region of the measurement spot2 a from which a certain percentage of the energy of the received heatradiation originates. Thus for example the inner circle 3 f couldrepresent the region of the measurement spot from which 90% of theenergy striking the detector originates. The second ring 3 g representsan energy value of 95% and the third ring 3 h would correspond to anenergy value of 99%. With the aid of such a light intensity distributionthe user can recognise the level of accuracy with which he can measureobjects of a certain size.

[0035] A further device according to the invention for temperaturemeasurement is represented in FIG. 3. The same reference numerals areused in this case for the same components. This second embodimentdiffers from the first one essentially in the design of the opticalsystem 4 and the optical element 5′c of the sighting arrangement 5. InFIG. 3 the optical element 5′c is constructed as an annular lens andaccordingly is designed to produce a light intensity distributionaccording to FIGS. 2a to 2 d. The infrared lens 4′b is arranged so thatit is surrounded by the annular lens 5′c. The detector 1 is thenprovided between the holographic element 5 b and the infrared lens 4′b.

[0036] Such an arrangement has the advantage that a beam divider can beomitted. However, a somewhat more complicated fixing of the detectormust be accepted, since the conically opening hologram 6 b must not berestricted thereby.

[0037] In the third embodiment illustrated in FIG. 4 the problem ofmounting the detector I is circumvented by providing the beam divider4′a between the holographic element 5 b and the arrangement consistingof the annular lens 5 c and the infrared lens 4′b. Thus the heatradiation emanating from the measurement spot 2 a is focussed first ofall by the infrared lens 4′b onto the beam divider 4′a and is theredeflected by 90° onto the detector 1.

[0038] Whereas all the previously described embodiments related todistant-focussed systems, an embodiment is shown in FIG. 5 in which theshape of the measurement spot of a close-focussed system can be renderedvisible with the aid of a diffractive optical system. In this case themeasurement plane, i.e., the object of measurement 2, lies directly inthe focus plane of the optical system 4. In each case two rays 3 i, 3 kdelimiting the infrared beam are shown in the drawing. The ray 3 iextends from the upper edge of the infrared lens 4′b to the upper edgeof the measurement spot 2 a or from the lower edge of the infrared lens4′b to the lower edge of the measurement spot. By contrast, the ray 3 kextends from the lower edge of the infrared lens 4′b to the upper edgeof the measurement spot 2 a or from the upper edge of the infrared lens4′b to the lower edge of the measurement spot.

[0039] The optical element 5′c of the sighting arrangement 5 is designedso as to produce two intensity cones 6 d and 6 e which substantiallyfollow the course of the marginal rays 3 k and 3 i. In this case theintensity cone 6 e describes the size of the measurement spot as far asthe focus plane and the intensity cone 6 d describes the divergentmeasurement spot after the focus plane.

[0040] A disadvantage of this embodiment is that the intensity cone 6 dextends inside the marginal ray 3 k, whilst the intensity cone 6 eextends outside the marginal ray 3 i. However, this disadvantage can beovercome by another design of the refracting and/or reflecting opticalelement 5′c.

[0041] In the embodiment according to FIG. 5 the light intensitydistribution could advantageously be formed by two circular concentricmarkings, wherein one circular marking identifies the measurement spotlying between the optical element 5′c and the focus plane and the othermarking identifies the measurement spot lying behind the focusplane—when viewed from the optical element.

1. Device for temperature measurement comprising: a) a detector forreceiving heat radiation (3) emanating from a measurement spot on anobject of measurement; b) an optical system for imaging the heatradiation emanating from the measurement spot onto the detector; c) anda sighting arrangement comprising: a laser: a diffractive opticalsystem, aligned to be illuminated by said laser, to produce adiffraction pattern in the form of light intensity distribution whichincludes a 0^(th) order point in the center and at least one intensivecircle displaced from the center; and an optical element, aligned to beilluminated by said intensive circle to position said intensive circlefor identifying and outlining the position and size of the measurementspot on the object of measurement by means of visible light and forpositioning the 0^(th) order pattern near the center of the measurementspot to facilitate sighting.